/a : Adds new files to the destination directory instead of replacing existing files.
You cannot use this command-line option with the /s or /u command-line option.
You cannot specify a file name for files you replace.
If you specify neither a drive nor a directory, replace uses the current drive and directory as the destination.
) piece of software Bulk Rename Utility, I was able to do this right from the context (right click) menu.